One of the most common processes for making polypropylene homopolymer and copolymer is based on what are commonly called loop reactors. In this process, a loop reactor is made up of a series of long, straight pipes (generally referred to as “legs”) which are connected by bent pipes to form a continuous loop. Pumps are incorporated into this loop to circulate the polymer and liquid monomer slurry. Propylene polymerization is an exothermic reaction so heat must be removed from the reactors. One method of efficient heat removal is to provide jacketed reactor legs, such as described in U.S. Publication No. 2007/0022768 A1 to Smith, where water flows through the jackets to control the reactor temperature by removing heat from the reactor slurry.
One type of reactor design for high capacity polymerization lines uses two of these loop reactors operating in series. That is, catalyst and liquid monomers are fed to the first reactor and the slurry that leaves that reactor along with more liquid monomers is fed to the second reactor. Slurry leaving the second reactor is sent on for monomer separation and recovery. In this design the maximum capacity of the line may be determined by the heat transfer area available in these reactors. For new plants it is desired to increase plant capacity to reduce the capital and operating cost per unit of production. Various small increases have been achieved by increasing the reactor operating temperature, decreasing the temperature of the circulating cooling water and increasing the circulation rates of the water used to remove heat from the reactors. However, no major increase in capacity above a nominal 400 to 450 kTons per year rate is possible in the current reactor design.
Also, the basic plant design with two series reactors has limitations in the capability to produce polymers of different molecular weights in the two reactors. This is a feature that is desired for some products. The slurry leaving the first loop reactor is a large portion of the feed to the second loop reactor. Therefore any hydrogen that is fed to the first loop reactor and not consumed in that reactor is automatically fed to the second reactor. Even if no additional hydrogen is added to the second reactor the concentration in that reactor will not be significantly different than in the first reactor. It is possible to only add hydrogen to the second of the two series loop reactors so that its hydrogen concentration is much higher than in the first reactor. However, the slurry leaving the second reactor is normally about 50% polypropylene polymer and 50% unreacted propylene/propane/hydrogen. After the polymer is separated, the unreacted propylene/propane/hydrogen is condensed, mixed with fresh propylene feed and that mixture is routed to both loop reactors. Because the unreacted propylene/propane/hydrogen from the second loop reactor contains unreacted hydrogen there will be some hydrogen in the mixture of fresh and recycle propylene streams and there will be some hydrogen entering the first loop reactor even if there is no additional hydrogen fed to that reactor. This limits the extent to which different molecular weight polymers can be made in the two reactors.
The inventor has solved these and other problems in aspects of the invention as described herein.